The use of satellite communications has increased the demand for circularly polarized antennas and for dual polarization antennas. For instance, many of the satellite transponders in use today carry two programs on the same frequency by using separate polarizations. Thus, single antenna structure may be called upon to simultaneously receive two polarizations, or perhaps to transmit in one polarization and receive in another. The single antenna structure must therefore separate the two polarization channels, to a high degree of isolation.
It is possible to have dual linear or dual circular polarization channel diversity. That is, a frequency may be reused if one channel is vertically polarized and the other horizontally polarized. Or, a frequency can also be reused if one channel uses right hand circular polarization (RHCP) and the other left hand circular polarization (LHCP). Polarization refers to the orientation of the E field in the radiated wave, and if the E field vector rotates in time, the wave is then said to be rotationally or circularly polarized.
Smaller, lighter, lower power receivers are now being developed to satisfy a variety of operational needs. For example, a small, lightweight, low-power, 4-channel satellite receiver (PCI computer card) will soon be fielded to meet the needs of many size-constrained platforms. Today, the antenna may be the only piece of associated equipment that remains to be miniaturized for use in various environments.
An electromagnetic wave (and radio wave, specifically) has an electric field that varies as a sine wave within a plane coincident with the line of propagation, and the same is true for the magnetic field. The electric and magnetic planes are perpendicular and their intersection is in the line of propagation of the wave. If the electric-field plane does not rotate (about the line of propagation) then the polarization is linear. If, as a function of time, the electric field plane (and therefore the magnetic field plane) rotates, then the polarization is rotational. Rotational polarization is in general elliptical, and if the rotation rate is constant at one complete cycle every wavelength, then the polarization is circular. The polarization of a transmitted radio wave is determined in general by the transmitting antenna (and feed)—by the type of the antenna and its orientation. For example, the monopole antenna and the dipole antenna are two common examples of antennas with linear polarization. A helix antenna is a common example of an antenna with circular polarization, and another example is a crossed array of dipoles fed in quadrature. Linear polarization is usually further characterized as either Vertical or Horizontal. Circular Polarization is usually further classified as either Right Hand or Left Hand.
The dipole antenna has been perhaps the most widely used of all the antenna types. It is of course possible however to radiate from a conductor which is not constructed in a straight line. Preferred antenna shapes are often Euclidian, being simple geometric shapes known through the ages. In general, antennas may be classified as charge separation or charge conveyance types, corresponding to dipoles and loops, and line and circle structures.
Circular polarization for dipole antennas has been attributed to George Brown, which was described in the literature as “The Turnstile Antenna”, Electronics, 9, 15, Apr. 1936. Approaches to circular polarization in loop antennas appear lesser known, or perhaps even unknown in the purest forms. For instance, the present edition “Antenna Engineering Handbook”, R. Johnson and H. Jasik editors, does not describe methods to obtain circular polarization from loop antennas. In spite of the higher gain of the full wave loop vs. the half wave dipole (3.6 dBi vs. 2.1 dBi), dipoles are commonly used for circular polarization needs, as for instance in turnstile arrays. Both the dipole turnstile and a single loop antenna are planar, in that their thin structure lies nearly in a single plane.
Many structures are described as loop antennas, but canonical loop antennas are a circle. The resonant loop is a full wave circumference circular conductor, often called a “full wave loop”. The typical prior art full wave loop is linearly polarized, having a radiation pattern that is a two petal rose, with two opposed lobes normal to the loop plane, and a gain of about 3.6 dBi. Reflectors are often used with the full wave loop antenna to obtain a unidirectional pattern.
Dual linear polarization (simultaneous vertical and horizontal polarization from the same antenna) has commonly been obtained from crossed dipole antennas. For instance, U.S. Pat. No. 1,892,221, to Runge, proposes a crossed dipole system. A dual polarized loop antenna could be more desirable however, as loops provide greater gain in smaller area. An existing, prior art approach to dual polarization in single loops does not come to mind.
U.S. Pat. No. 5,977,921 to Niccolai, et al. and entitled “Circular-polarized Two-way Antenna” is directed to an antenna for transmitting and receiving circularly polarized electromagnetic radiation which is configurable to either right-hand or left-hand circular polarization. The antenna has a conductive ground plane and a circular closed conductive loop spaced from the plane, i.e., no discontinuities exist in the circular loop structure. A signal transmission line is electrically coupled to the loop at a first point and a probe is electrically coupled to the loop at a spaced-apart second point. This antenna requires a ground plane and includes a parallel feed structure, such that the RF potentials are applied between the loop and the ground plane. The “loop” and the ground plane are actually dipole half elements to each other, and the invention is related to microstrip antennas.
U.S. Pat. No. 5,838,283 to Nakano and entitled “loop antenna for radiating circularly polarized waves” is directed to a loop antenna for a circularly polarized wave. Driving power fed may be conveyed to a feeding point via an internal coaxial line and a feeder conductor is transmitted through an I-shape conductor to a C-type loop element disposed in spaced facing relation to a ground plane. By the action of a cutoff part formed on the C-type loop element, the C-type loop element radiates a circularly polarized wave. Dual linear, or dual circular polarization are not however provided.
U.S. Pat. No. 6,522,302 to Iwasaki and entitled “circularly-polarized antennas” is directed to a circularly polarized antenna array rather than a single circularly polarized loop element. A circle is among the most elemental of antenna structures, and it is the most fundamental single geometry capable of circular polarization.
There is a longstanding requirement then, to obtain circular polarization from a single loop antenna, such as the full wave circumference circular conductor, and to identify a polarization method for loops dual to the turnstile for dipoles. A method is also needed, to obtain dual linear or dual circular polarization from a single loop antenna. Finally, there has been a practical need for a relatively compact loop antenna with dual polarization, linear or circular, such as to meet the requirements of today's multiplexed satellite communications.